The ins and outs of APC and beta-catenin nuclear transport

Adenomatous polyposis coli (APC) and β-catenin, two key interacting proteins implicated in development and cancer, were recently found to traffic into and out of the nucleus in response to internal and external signals. The two proteins can enter and exit the nucleus independently, a discovery that has prompted debate about the previously proposed role of APC as a β-catenin chaperone. Here, we review the regulation of APC and β-catenin subcellular localization, in particular in cancer cells. We speculate that, in non-stimulated cells, APC actively exports β-catenin from the nucleus to the cytoplasm where its levels are regulated by degradation; and, conversely, that, in cancer cells or those stimulated by Wnt signaling, β-catenin degradation is inhibited and the accruing protein is capable of moving between the nucleus and cytoplasm independently of APC. Models that link APC and β-catenin transport to function are discussed.